2. Composition of MatterComposition of Matter  Matter - Everything in universe is composed of matter  Matter is anything that occupies space or has mass  Mass – quantity of matter an object has  Weight – pull of gravity on an object

3. Elements  Pure substances that cannot be broken down chemically into simpler kinds of matter  More than 100 elements (92 naturally occurring)

4.  90% of the mass of an organism is composed of 4 elements (oxygen, carbon, hydrogen and nitrogen)  Each element unique chemical symbol  Consists of 1-2 letters  First letter is always capitalized

5. Atoms  The simplest particle of an element that retains all the properties of that element  Properties of atoms determine the structure and properties of the matter they compose  Our understanding of the structure of atoms based on scientific models, not observation

6. The Nucleus  Central core  Consists of positive charged protons and neutral neutrons  Positively charged  Contains most of the mass of the atom

7. The Protons  All atoms of a given element have the same number of protons  Number of protons called the atomic number  Number of protons balanced by an equal number of negatively charged electrons

8. The Neutrons  The number varies slightly among atoms of the same element  Different number of neutrons produces isotopes of the same element

9. Atomic MassAtomic Mass  Protons & neutrons are found in the nucleus of an atom  Protons and neutrons each have a mass of 1 amu (atomic mass unit)  The atomic mass of an atom is found by adding the number of protons & neutrons in an atom

10. The Electrons  Negatively charged high energy particles with little or no mass  Travel at very high speeds at various distances (energy levels) from the nucleus

12. Compounds  Most elements do not exist by themselves  Readily combine with other elements in a predictable fashion

13.  A compound is a pure substance made up of atoms of two or more elements  The proportion of atoms are always fixed  Chemical formula shows the kind and proportion of atoms of each element that occurs in a particular compound

14.  Molecules are the simplest part of a substance that retains all of the properties of the substance and exists in a free state  Some molecules are large and complex

15. Chemical Formulas  Subscript after a symbol tell the number of atoms of each element  H 2 0 has 2 atoms of hydrogen & 1 atom of oxygen  Coefficients before a formula tell the number of molecules  3O 2 represents 3 molecules of oxygen or (3x2) or 6 atoms of oxygen

16. Covalent BondsCovalent Bonds  Formed when two atoms share one or more pairs of electrons

17. Ionic BondsIonic Bonds  Some atoms become stable by losing or gaining electrons  Atoms that lose electrons are called positive ions

18.  Atoms that gain electrons are called negative ions  Because positive and negative electrical charges attract each other ionic bonds form